Power amplifier having protective circuits

Abstract

An improved circuit is provided for a known amplifier having complementary input transistors in its power output stages for supplying oppositely phased signals in response to an input signal. Output transistors are connected to the input transistors to provide output signals for the amplifier. Normally conducting switching means are connected between one of the output transistors and one of the input transistors. Normally nonconducting switching means are coupled between the one output transistor and the amplifier input. During an overload or overdrive condition, the normally conducting switching means become nonconducting, and this in turn causes the normally nonconducting switching means to become conducting, and thereby limit the output from the amplifier. The improved circuit for the amplifier includes a threshold device which responds to the normally conducting switching means becoming nonconducting and reduces the input signal amplitude, and thereby permits the amplifier to restore itself when the overload or overdrive condition is removed.

What we claim as new and desire to secure by Letters Patent of the United States is: 
     
       1. In a transistorized power amplifier comprising: a. an input for said amplifier;   b. a pair of complementary input transistors connected to said input to supply oppositely phased signals in response to an input signal applied to said amplifier input;   c. a pair of output transistors having input circuits respectively connected to said complementary input transistors, said output transistors being responsive respectively to said oppositely phased signals to provide output signals for said amplifier;   d. normally conductive switching means connected between one of said output transistors and one of said complementary input transistors;   e. normally nonconductive switching means connected between said one output transistor and said amplifier input;   f. said normally conductive switching means becoming nonconductive in response to an overload or an overdrive applied to said amplifier, and said normally nonconductive switching means becoming conductive in response to said normally conductive switching means becoming nonconductive; an improved restoral circuit comprising:   g. a threshold device having an input connected to said one output transistor and having an output that produces a threshold signal in response to a predetermined voltage at its input;   h. and an attenuator connected to said amplifier input, said attenuator having a control input connected to said threshold output for reducing the magnitude of input signals applied to said amplifier input in response to said threshold signal.   
     
     
       2. A transistorized power amplifier comprising: a. an input for said amplifier;   b. a first pair of complementary input transistors connected to said amplifier input to supply oppositely phased signals in response to an input signal applied to said amplifier input;   c. a second pair of complementary input transistors connected to said amplifier input to supply oppositely phased signals in response to an input signal applied to said amplifier input;   d. a first pair of output transistors connected in series and having input circuits respectively connected to said first pair of complementary input transistors, said first pair of output transistors being responsive respectively to oppositely phased signals to provide output signals for said amplifier;   e. a second pair of output transistors connected in series and having input circuits respectively connected to said second pair of complementary input transistors, said second pair of output transistors being responsive respectively to said oppositely phased signals to provide output signals for said amplifier;   f. first normally conductive switching means connected between one of said first pair of output transistors and one of said first pair of complementary input transistors;   g. second normally conductive switching means connected between one of said second pair of output transistors and one of said second pair of complementary input transistors;   h. first normally nonconductive switching means connected between said one of said first pair of output transistors and said amplifier input;   i. second normally nonconductive switching means connected between said one of said second pair of output transistors and said amplifier input;   j. said normally conductive switching means becoming nonconductive in response to an overload or an overdrive applied to said amplifier, and said normally nonconductive switching means becoming conductive in response to said normally conductive switching means becoming nonconductive;   k. a threshold device having an input connected to said one output transistor of said first and second pairs of output transistors and having an output that produces a threshold signal in response to a selected voltage at its input;   l. and an attenuator connected to said amplifier input, said attenuator having a control input connected to said threshold device output for reducing the magnitude of input signals applied to said amplifier in response to said threshold signal.   
     
     
       3. The amplifier of claim 2 and further comprising: an output transformer connected between the series junction of said first pair of output transistors and the series junction of said second pair of output transistors, and wherein said threshold device input is connected to a center tap of said output transformer. 
     
     
       4. The amplifier of claim 3, and further comprising a device for detecting alternating current at said output transformer center tap for indicating failure. 
     
     
       5. In a transistor amplifier comprising: a. a driver stage including a pair of transistors having a common signal input point and responsive to signals applied thereto to produce a pair of oppositely phased output signals;   b. an output stage including a pair of series connected class B transistors having an output load point at a junction therebetween and each having an input connected to one of said driver stages respectively for producing an output signal at said output load point in response to said oppositely phased signals produced by said driver stage;   c. a protective circuit comprising: 1. a first diode circuit having its anode biased by a positive potential and its cathode biased by a negative potential;   2. means connecting said first diode circuit cathode to said output load point;   3. a second diode circuit having its anode connected to said output load point and its cathode connected to said common signal input point of driver stage; an improved restoral circuit comprising:     d. a threshold device having an input connected to said output load point and having an output for producing a threshold signal in response to said output load point reaching a predetermined positive voltage magnitude;   e. and an attenuator connected to said common input point of said driver stage, said attenuator having a control connected to said threshold device for reducing the amplitude of input signals at said common input point of said driver stage to a predetermined low magnitude in response to threshold signals.   
     
     
       6. A transistor amplifier comprising: a. a first driver stage including a pair of transistors having a common signal input point and responsive to signals applied thereto to produce a pair of oppositely phased output signals;   b. a second driver stage including a pair of transistors having a common signal input point and responsive to signals applied thereto to produce a pair of oppositely phased output signals;   c. a first output stage including a pair of series connected transistors having a first output load point at a junction therebetween and each having an input connected to one of said transistors respectively of said first driver stage for producing an output signal at said first output load point in response to said oppositely phased signals produced by said first driver stage;   d. a second output stage including a pair of series connected transistors having a second output load point at a junction therebetween and each having an input connected to one of said transistors respectively of said second driver stage for producing an output signal at said second output load point in response to said oppositely phased signals produced by said second driver stage;   e. a first protective circuit comprising: 1. a first diode circuit havings it anode biased by a positive potential and its cathode biased by a negative potential;   2. means connecting said first diode circuit cathode to said first output load point of said first output stage;   3. a second diode circuit having its anode connected to said first output load point of said first output stage and its cathode connected to said common input point of said first driver stage;     f. a second protective circuit comprising: 1. a third diode circuit having its anode biased by a positive potential and its cathode biased by a negative potential;   2. means connecting said third diode circuit cathode to said second output load point of said second output stage;   3. a fourth diode circuit having its anode connected to said second output load point of said second output stage and its cathode connected to said common input point of said second driver stage;     g. a threshold device having an input connected to said first and second output load points of said first and second output stages and having an output for producing a threshold signal in response to at least one of said output load points reaching a predetermined positive voltage magnitude;   h. and an attenuator connected to said common input points of said first and second driver stages, said attenuator having a control input connected to said threshold device for reducing the magnitude of input signals at said common input points of said driver stages to a predetermined low magnitude in response to threshold signals.   
     
     
       7. The transistor amplifier of claim 6, and further comprising an output transformer connected between said first output load point of said first output stage and said second output load point of said second output stage, and wherein said threshold device input is connected to a center tap of said output transformer. 
     
     
       8. The transistor amplifier of claim 7, and further comprising a device for detecting alternating current at said output transformer center tap for indicating failure of said amplifier. 
     
     
       9. The transistor amplifier of claim 8, and further comprising a varistor and inductor circuit connected to said output transformer for providing protection and stability to said transistor amplifier, and a current sensing circuit connected between said output transformer and said common input points of said driver stages for adjusting the output impedance of said transistor amplifier.